feat: implement UI config flow with single-instance enforcement

custom_components/nestquest/config_flow.py

@@ -0,0 +1,31 @@
+"""Config flow for NestQuest."""
+from __future__ import annotations
+
+from typing import Any
+
+from homeassistant import config_entries
+from homeassistant.data_entry_flow import FlowResult
+
+from .const import DOMAIN
+
+
+class NestQuestConfigFlow(config_entries.ConfigFlow, domain=DOMAIN):
+    """Handle a config flow for NestQuest."""
+
+    VERSION = 1
+
+    async def async_step_user(
+        self, user_input: dict[str, Any] | None = None
+    ) -> FlowResult:
+        """Handle the user step of the config flow."""
+        existing = any(
+            entry.domain == DOMAIN
+            for entry in self.hass.config_entries.async_entries(DOMAIN)
+        )
+        if existing:
+            return self.async_abort(reason="single_instance_allowed")
+
+        if user_input is None:
+            return self.async_show_form(step_id="user")
+
+        return self.async_create_entry(title="NestQuest", data=user_input)